1. Field of the Invention
This invention relates to optical fibers and more particularly to photonic bandgap fibers.
2. Description of the Related Art
The concept of optical waveguides based on photonic bandgap (PBG) in periodic optical media was first proposed in a theoretical paper by Yeh and Yariv in 1978 (“Theory of Bragg Fibers”, Journal of Optical Society of America, vol. 68, no. 9, September 1978, pp. 1196–1201). Not until 21 years thereafter was the first practical demonstration of an optical fiber guided by the PBG effect reported in a paper by Cregan et al published in Science in September 1999 (R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. St. J. Russell, P. J. Roberts, and D. C. Allan: “Single-mode Photonic Bandgap Guidance of Light in Air”, Science, vol. 285, September 1999, pp. 1537–1539). In these first demonstrations, the cladding of the optical fiber was formed by triangular stacking of silica capillaries and the core was formed by a central large air hole. The cladding of this fiber was not, in cross-section, a set of concentric circles of different mediums as proposed in the original 1978 paper by Yeh and Yariv, which is referred to as Bragg fiber. The same principles, however, form the basis of both waveguides. A first Bragg fiber demonstration was reported in November 1999 by Fink in a paper published in Journal of Lightwaves Technology (Y. Fink, D. J. Ripin, S. Fan, C. Chen, J. D. Joannopoulos, and E. L. Thomas: “Guiding Optical Light in Air Using an All-Dielectric Structure”, Journal of Lightwaves Technology, vol. 17, no. 11, November 1999, pp. 2039–2041).
Since the first demonstration of the photonic bandgap fibers (PBGF) in 1999, progress has been swift. Smith et al reported PBGF with loss as low as 13 dB/km in a paper published in Nature in August 2003 (C. M. Smith, N. Venkataraman, M. T. Gallagher, D. Muller, J. A. West, N. F. Borrelli, D. C. Allan, and K. W. Koch: “Low-loss Hollow-core Silica/air Photonic bandgap Fiber”, Nature, vol. 424, August 2004, pp. 657–659). A further breakthrough came in a post-deadline paper at the Optical Fiber Communications Conference in February 2004 (B. J. Mangan, L. Farr, A. Langford, P. J. Roberts, D. P. Williams, F. Couny, M. Lawman, M. Mason, S. Coupland, R. Flea, and H. Sabert: “Low Loss (1.7 dB/km) Hollow Core Photonic Bandgap Fiber”, PDP24, Optical Communications Conference, February 2004). Mangan et al reported a PBGF with loss as low as 1.7 dB/km.
This progress has brought the technology closer to real world applications. A first area of application is high energy optical pulse propagation. In general, most of the optical power propagating along the optical fiber is in the core, which typically comprises a hole in the center of the PBGF. Light can effectively propagate in vacuum, air, or inert gas with much lower nonlinear coefficients than solids. Accordingly, such hollow cores are an ideal media to propagate optical pulses with high peak power. Such pulses may not otherwise be guided over substantial distances in a conventional optical fiber due to pulse distortion and/or energy loss from nonlinear processes in the core glass. A first demonstration of such high peak power pulse propagation was reported in a paper in Science published in 2003 by Ouzounov et al (D. G. Ouzounov, F. R. Ahmad, A. L. Gaeta, D. Muller, N. Venkataraman, M. Gallagher, C. M. Smith, and K. W. Koch, Science, vol. 301, 2003, pp. 1702). Xenon gas was used to fill the core during one of the reported experiments. Distortion-free transmission over 100 m with pulse intensities up to 1013 W/cm2 was achieved.
Accurate dispersion control is useful for optical fibers employed for long haul transmission and pulse shaping. In the absence of nonlinearity, dispersion dictates the pulse evolution during transmission through the optical fiber. In cases where the pulse shape is to be preserved, e.g. in telecommunications and delivery of optical pulses, low dispersion may be desirable. In particular, a flat low dispersion over a wide bandwidth can be helpful. A notable example is wavelength-division-multiplexing in telecommunication where a constant low dispersion level over the wavelength can help provide a uniform performance for all carrier wavelengths. Conversely, in cases where a pre-determined level of pulse shaping is desirable, a high level of dispersion with controllable amount of variation over wavelength may be preferred instead. A notable example is pulse compression in a high energy pulse system, where a combination of second and third order dispersion (β2 and β3, where βm=dmβ/dωm, and, β and ω are propagation constant and optical frequency) can be used to achieve a fair amount of compensation.
What is needed therefore is the ability to design optical fibers having the desired dispersion characteristics.